1. Field of the Invention
The present invention relates in general to high frequency induction heating appliances, and more particularly to a high frequency induction heating appliance for sensing accurately the amount of load, such as a size of a container to be heated.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a block diagram of a conventional high frequency induction heating appliance. As shown in this figure, the conventional appliance comprises an alternating current (AC) power supply 1, a bridge diode 2 for full-wave rectifying a commercial AC power (for example, 110 V or 220 V) from the AC power supply 1, a smoothing circuit 3 for smoothing the full-wave rectified direct current (PC) voltage from the bridge diode 2, a working coil 4 being driven by the smoothed DC voltage from the smoothing circuit 3 being applied as a drive voltage thereto to generate a high frequency induction heating signal, a resonance condenser C3 connected between both sides of the working coil 4, a NPN type bipolar transistor Q1 connected between the working coil 4 and the ground for switching the smoothed DC voltage from the smoothing circuit 3 being applied as the drive voltage to the working coil 4, a current transformer 5 connected to the AC power supply 1 for detecting current of the AC power supply 1, a voltage generator 6 for generating a voltage corresponding to the detected current from the current transformer 5, an output adjusting device 7 for providing a signal for adjusting output of the appliance under adjustment of the user, and an operational amplifier 8 for amplifying a difference between an output signal from the voltage generator 6 inputted at its inverting input terminal (-) and an output signet from the output adjusting device 7 inputted at its non-inverting input terminal (+).
The conventional high frequency induction heating appliance also comprises a first comparator 9 for comparing the output signal from the voltage generator 6 inputted at its inverting input terminal (-) with a predetermined reference voltage from a reference voltage generator 9a inputted at its non-inverting input terminal (+), an integrator 10 being disabled or enabled by an output signal from the first comparator 9 to integrate an output signal from the operational amplifier 8 upon being enabled, a start signal generating circuit 11 being disabled or enabled in the opposite manner to the integrator 10 by the output signal from the first comparator 9 to generate a start signal for starting the operation of the appliance upon being enabled, a diode D1 for blocking a minus voltage of an output signal from the integrator 10, a diode D2 for blocking a minus voltage of an output signal from the start signal generating circuit 11, a synchronizing signal generator 12 for generating a synchronizing signal corresponding to the driven state of the working coil 4, a reference signal generator 13 for generating a saw tooth wave signal as in reference signal whenever the synchronizing signal is generated from the synchronizing signal generator 12, a second comparator 14 for comparing an output signal from the diode D1 or an output signal from the diode D2 inputted at its non-inverting input terminal (+) with the saw tooth wave signal from the reference signal generator 13 inputted at its inverting input terminal (-), and a drive controller 15 for controlling turning on/off of the NPN type bipolar transistor Q1 in response to an output signal from the second comparator 14.
Now, the operation of the conventional high frequency induction heating appliance with the above-mentioned construction will be described.
First, the high frequency induction heating appliance is applied with the commercial AC power (110 V/220 V) and the output adjusting device 7 therein generates the output adjusting signal under the adjustment of the user. Since the current inputted at the voltage generator 6 through the current transformer 5 is zero at the initial state in which the power is applied to the appliance, the operational amplifier 8 amplifies only the output adjusting signal from the output adjusting device 7. At the same time, the first comparator 9 outputs a high signal because its inverting input terminal is applied with the zero voltage from the voltage generator 6 and its non-inverting input is applied with a voltage from reference voltage generator 9a. The high signal from the first comparator 9 is applied as an enable signal to the start signal generating circuit 11 and as a disable signal to the integrator 10. As a result, the integrator 10 performs no integration of the output signal from the operational amplifier 8 and the start signal generating circuit 11 outputs the start signal for starting the operation of the high frequency induction heating appliance. The start signal from the start signal generating Circuit 11 is applied through the diode D2 to the non-inverting input terminal (+) of the second comparator 14.
On the other hand, the synchronous signal generator 12 is connected between both the sides of the working coil 4 to generate a synchronizing signal corresponding to a variation of the drive voltage being applied to the working coil 4. The drive voltage to the working coil 4 is varied according to the switching (turning-on/off) of the NPN type bipolar transistor Q1.
The reference signal generator 13 generates a saw tooth wave signal as the reference signal whenever the synchronizing signal is generated from the synchronizing signal generator 12. The generated saw tooth wave signal is applied to the inverting input terminal (-) of the second comparator 14. At the initial operating state of the high frequency induction heating appliance, the second comparator 14 outputs a high signal enabling the drive controller 15 to turn on the NPN type bipolar transistor Q1.
Thereafter, the voltage generator 6 outputs a voltage corresponding to the detected current from the current transformer 5. As a result, the operational amplifier 8 amplifies a difference between the output signals from the output adjusting device 7 and the voltage generator 6. At this time, the first comparator 9 outputs a low signal when the output voltage from the voltage generator 6 applied to its inverting input terminal (-) is greater than the predetermined reference voltage applied to its non-inverting input terminal (+). The low signal from the first comparator 9 is applied as an enable signal to the integrator 10 and as a disable signal to the start signal generating circuit 11. As a result, the integrator 10 integrates the output signal from the operational amplifier 8 and feeds the integrated value through the diode D1 to the non-inverting input terminal (+) of the second comparator 14. In this connection, the integrated value is varied based on the output adjusting signal which is applied from the output adjusting device 7 to the non-inverting input terminal (+) of the operational amplifier 8 under the adjustment of the user.
The second comparator 14 compares the output signal from the integrator 10 with the output signal from the reference signal generator 13 and outputs a high or low signal to the drive controller 15 as a result of the comparison. In response to the high or low signal from the second comparator 14, the drive controller 15 turns on or off the NPN type bipolar transistor Q1.
When the NPN type bipolar transistor Q1 is turned on, the output signal from the smoothing circuit 3 is applied as the drive voltage to the working coil 4, thereby allowing the working coil 4 to generate the high frequency induction heating signal. The resonance condenser C3 is adapted to resonate the energy accumulated in the working coil 4 when the NPN type bipolar transistor Q1 is turned off, so as to lower the high DC voltage applied to the working coil 4.
However, the conventional high frequency induction heating appliance has a disadvantage, in that the operation thereof is started regardless of the waveform of the commercial AC power inputted therein. For this reason, there is quite a possibility that a load sensing circuit (not shown) which is connected to the AC power supply 1 together with the current transformer 51 may misjudge the amount of load such as a size of a container to be heated. Generally, when a large container is put on the working coil 4, the load amount (resistance) becomes large and the voltage from the AC power supply 1 thus becomes large. As a result, large current flows from the AC power supply 1, thereby causing the load sensing circuit to judge that a large container has been put on the working coil 4. On the contrary, when a small container is put on the working coil 4, the load amount becomes small and the voltage from the AC power supply 1 thus becomes small. As a result, small current flows from the AC power supply 1, thereby causing the load sensing circuit to judge that a small container has been put on the working coil 4. For example, when the peak value of the commercial AC power is applied as shown as a point P1 in FIG. 2 under the condition that the small container is put on the working coil 4 and the high frequency induction heating appliance is initially operated based on the generation of the start signal, the high frequency signal from the working coil 4 is directly generated and large current flows from the AC power supply 1. This causes the load sensing circuit to misjudge that a large container has been on the working coil 4. Alternatively, when the value of the commercial AC power applied is near the zero point as shown as a point P2 in FIG. 2 under the condition that a large container is put on the working coil 4 and the high frequency induction heating appliance is initially operated based on the generation of the start signal, the working coil 4 is not driven and no current flows from the AC power supply 1. This causes the load sensing circuit to misjudge that no container is put on the working coil 4. Further, when a value of the commercial AC power applied is near the zero point under the condition that the NPN type bipolar transistor Q1 is turned on based on the generation of the start signal without respect to the presence of the container on the working coil, the working coil 4 is not driven and the load sensing circuit cannot discriminate the presence of the container on the working coil 4.